1. Technical Field
The present invention embodiments pertain to devices producing optical signals. In particular, present invention embodiments pertain to an optical parametric oscillator (OPO) providing a narrow linewidth (e.g., optical spectrum width or band) over a tunable wavelength range and enhanced power output. The optical parametric oscillator (OPO) may be employed for detection of chemicals (e.g., narrow-line and broad feature (or wider-line)) and/or biological aerosols.
2. Discussion of Related Art
Optical parametric oscillators (OPO) are well-known, non-linear optical devices capable of producing coherent radiation at a desired frequency via parametric amplification. In a conventional optical parametric oscillator (OPO), a pump source supplies a beam of laser light at a pump wavelength to an optical cavity bounded by end mirrors and containing a non-linear optical medium (typically a non-linear optical crystal). As the pump beam propagates through the non-linear optical medium within the optical cavity, photons at the pump wavelength are converted into photon pairs at two longer wavelengths, thereby resulting in two lower-energy beams with these two longer wavelengths (conventionally referred to as the signal wavelength and the idler wavelength). The sum of the frequencies of the signal and idler beams (having the respective signal and idler wavelengths) equals the frequency of the pump beam. The particular wavelengths of the signal and idler beams are determined by a number of factors, including: the pump wavelength, the type and cut of the non-linear optical crystal, and the design of the optical cavity. In addition, tuning of the signal and idler beams can be achieved by adjusting the angle of the non-linear optical crystal.
Since typical operating conditions cause only a small fraction of the pump beam to be converted to the signal and idler beams in the initial pass through the non-linear optical crystal, the optical cavity of the optical parametric oscillator (OPO) is generally designed to oscillate one or both of the parametrically generated beams such that the signal and/or idler beam is amplified in successive passes through the non-linear optical crystal. The optical parametric oscillator (OPO) is considered a doubly resonant oscillator when both of the generated optical beams are resonated, and is considered a singly resonant oscillator when only one of the generated optical beams is resonated. Specifically, the optical cavity can be designed with end mirrors that reflect only one of the signal and idler frequencies (singly resonant), or with end mirrors that reflect both the signal and idler frequencies (doubly resonant).
An application of an optical parametric oscillator (OPO) is a light source for detection and/or identification of chemical/biological entities. With respect to remote sensing in the mid-wavelength infrared (MWIR) region, a wide variety of chemicals of interest have unique absorption features. One approach to sensing capitalizes on this characteristic by using a widely tunable (i.e., 3.1-3.6 microns) light source for both Differential Absorption Lidar (DIAL) and Differential Scattering (DISC) applications. However, the large tuning range of the light source prevents the optical parametric oscillator (OPO) from achieving linewidths (e.g., optical spectrum width or band) significantly lower than 300 picometers (pm), which is similar to the width of many narrow-line chemicals of interest (e.g., Hydrogen Chloride (HCl), etc.). Further, these narrow linewidths are not conducive for optimum measurements since an extremely tight tolerance is mandated on the central wavelength.
The above described oscillator functions adequately for chemicals with broad features requiring a broader linewidth (e.g., optical spectrum width or band) (e.g., dimethyl methylphosphonate (DMMP), triethyl phosphate (TEP), etc.). In order to provide the wide tuning range and the capability of measuring chemicals with either broad or narrow absorption features, a common solution employs a dual leg system. A first leg of the system contains an optical parametric oscillator (OPO) to perform measurements of chemicals with broad features, while the second leg of the system contains a narrow linewidth optical parametric amplifier (OPA) to measure the narrow-line chemicals. However, this system is impractical for the measurement of arbitrary chemicals. In particular, the optical parametric amplifier (OPA) requires seed lasers to generate the narrow linewidth, where the seed laser wavelengths are different for each chemical. Since a system to accommodate several chemicals requires a large number of these seed lasers, this type of system is extremely difficult to construct and expensive.